Conventionally, there is known a DC power supply serving as a DC power source for various electronic devices, as disclosed in, e.g., Japanese Patent Application Publication No. 2001-178137. The disclosed DC power supply, as illustrated in FIG. 8, includes mainly an AC power source 100, a full-wave rectifier 101 for rectifying an AC power source, and a step-up chopper circuit 102 for stepping up the voltage rectified by the full-wave rectifier 101. Further, a capacitor 103 is provided between the full-wave rectifier 101 and the step-up chopper circuit 102, thereby removing noise.
The step-up chopper circuit 102 includes, as main components, a choke coil 102A, a diode 102B, a switching element 102C, a control circuit 102D for controlling an ON/OFF interval of the switching element 102C, and an electrolytic capacitor 102E for smoothing an output. Further, in the DC power supply shown in FIG. 8, a DC voltage obtained by rectifying the AC voltage by the full-wave rectifier 101 is stepped up to a predetermined voltage by the step-up chopper circuit 102.
In the conventional example, there is further provided an inrush current prevention circuit 104 for limiting an inrush current occurring when the AC power source 100 is turned on. As the inrush current prevention circuit 104, there is used a circuit in which a resistor 104A and a thyristor 104B are connected in parallel at an output side of the full-wave rectifier 101. Further, a positive voltage is applied between the gate and cathode of the thyristor 104B from one winding of the choke coil 102A included in the step-up chopper circuit 102.
The inrush current prevention circuit 104 is operated as follows. First, the AC power source 100 is turned on to operate the DC power supply. At this point, since no drive voltage is applied between the gate and cathode of the thyristor 104B, all of the current flowing into the inrush current prevention circuit 104 is supplied to the step-up chopper circuit 102 via the resistor 104A.
Further, when a voltage is applied to the step-up chopper circuit 102, the switching element 102C is turned on by the control circuit 102D and the energy begins to accumulate in the choke coil 102A. At this point, the thyristor 104B starts to operate and a path of the current flowing through the inrush current prevention circuit 104 is switched from a path passing through the resistor 104A to a path passing through the thyristor 104B.
Furthermore, in a case where a voltage is applied to a light source including a light emitting diode by using the DC power supply as described in the above conventional example, it requires a step-down chopper circuit for stepping down an output voltage of the DC power supply before supplying the voltage. The step-down chopper circuit generally includes a switching element such that the output voltage of the DC power supply is stepped down by switching ON/OFF of the switching element.
In this case, when a short circuit failure occurs in the switching element, an overcurrent flows in the light source. Further, if the light source is destroyed due to the overcurrent, there is no problem even if the output voltage of the DC power supply is continuously applied between both ends of the light source. However, if the light source is not destroyed, the overcurrent continuously flows through the light source. Accordingly, the light source generates heat and a member or the like arranged around the light source increases in temperature, thereby causing an adverse effect such as melting of the member.
In an LED downlight using a light source constituted by, e.g., a light emitting diode, a reflecting plate is placed around the light source. The reflecting plate is made of various materials. If the reflecting plate is made of polybutylene terephthalate (PBT), its melting temperature is about 230° C. As described above, if the light source generates heat and a temperature therearound reaches the melting temperature, the reflecting plate is melted and deformed, which may result in a failure, e.g., failing to obtain desired light distribution characteristics.